1. Field of the Invention
The present invention is directed generally to a method for handling a header field of message cells, or packets, transmitted in the asynchronous transfer mode (ATM) in the course of through-connection through a switching network.
2. Description of the Related Art
When handling message cells in a switching network of a an ATM (asynchronous transfer mode) switching center, a first component part VPI of the header field serves to identify a traffic route for the message cell and a second component part VCI of the header field serves to identify the connection, or output, port. These two components in the header fields of incoming ATM messages cells are converted into corresponding components in the header fields of outgoing ATM message cells. The two components VPI and VCI of the header generally span 28 bits within the header field, so that there are over 256 million possible combinations for these two components.
To avoid difficulties which would result from such a great number of possible bit combinations, a limitation on the routing and output number of possible combinations is imposed by limiting the information to 12 bits, which reduces the possible combinations to just over 4000. This limitation of the two header components is achieved through the use of an associative memory before implementation of the conversion of the first and second component parts VPI and VCI of incoming ATM message cells into corresponding component parts of outgoing ATM message cells. The conversion at the output need only be implemented using this limited number of bits.
For through-connection of packets through an ATM switching center, the header field of the ATM message cells is supplemented by additional information. The header field which is present outside the switching center is termed the external header field, while the additional portion added during switching is known as an internal header field. The internal header field includes, among other things, routing information for defining the route through the switching network, as well as a cell sequence number.
It has heretofore been true that the steps of header field combination/reduction, of conversion, of attaching an internal header field, and of subsequent restoration of an original number of bits to the header field components VPI and VCI were implemented in peripheral equipment at an input side of the switching center.
To distribute the traffic load as uniformly as possible in an ATM switching network, particularly given utilization of a flexible bandwidth allocation and to avoid overloading of individual internal routes, all routes leading from a specific switching network input to a specific switching network output are utilized for the through-connection of ATM message cells belonging to one and the same call. As a result, corresponding different routing information is inserted into the internal header portion cyclically interchanged. This method is also known as multi-pathing. As a consequence of the use of different routes, however, differences in transit time occur so that, in certain circumstances, the original cell sequence may be lost. For this reason, a restoration of the original cell sequence must be undertaken at the switching network output by using the cell number in the internal header field.
This so-called "resequencing" must be individually carried out for every connection, in other words, for every possible combination of first and second components VPI and VCI. Corresponding technical difficulties would arise if this resequencing were undertaken with the full 28 bits of the two components VPI and VCI of the external header portion. These technical difficulties are similar to those in the conversion of the possible component VPI and VCI combinations for incoming ATM message cells into corresponding combinations of outgoing ATM message cells.